Glass-ceramic materials comprise an amorphous (glass) phase and one or more crystalline (ceramic) phases embedded in the amorphous phase. Due to the presence of both an amorphous and a crystalline phase, glass-ceramics share many properties with both glasses and ceramics. They are used in a variety of different technical fields, for example as cooktops, cookware and bakeware, as a substrate for magnetic disks, as high performance reflectors for digital projectors or as prosthetic elements.
Glass-ceramics are of particular interest in the field of restorative dentistry, since there is a high demand for prosthetic elements which exhibit excellent characteristics comparable to those of natural teeth in terms of functionality and appearance.
Prosthetic elements for use in dental restorations, such as crowns, bridges, abutments, onlays and inlays, are today increasingly manufactured by computer aided design/computer aided machining (CAD/CAM) technologies. The fabrication process comprises two decisive stages: a computer-aided design of the restoration and its computer-aided milling. In the stage of milling, the restoration is machined out of a dental material block.
In view of the preparation of a prosthetic element by a CAD/CAM method, it is essential that the material on which the element is based not only has appealing optical properties as well as a high strength and chemical durability. The material should also allow to be machined into the desired shape in an easy manner without undue wear of the tools.
A process for preparing a shaped glass-ceramic dental product comprising the step of machining a respective blank is e.g. proposed in DE-A-19750794. More particularly, DE-A-19750794 relates to a lithium disilicate glass-ceramic and is aiming at a high chemical stability, a high translucency and good mechanical properties of the product. Due to the high strength and toughness imparted by the lithium disilicate crystalline phase, however, it has been shown that application of this method results in increased tool wear during finishing and insufficient edge strength of the restorations prepared especially when their thickness falls within a range of only a few hundreds of micrometers.
U.S. Pat. No. 7,452,836 relates to a process for providing a glass-ceramic which has metastable lithium metasilicate (Li2SiO3) as main crystalline phase. This lithium metasilicate glass-ceramic has mechanical properties allowing it to be easily machined into the shape of even complicated dental restorations without undue wear of tools. It can be converted by further heat treatment into a lithium disilicate glass-ceramic with very good mechanical properties and translucency.
Although U.S. Pat. No. 7,452,836 allows for achieving materials having a flexural strength which might be sufficient for the restoration of multiple missing teeth in the front (3-unit bridges), its strength is still not sufficient for posterior bridges or large restorations.
A further shortcoming of the method according to U.S. Pat. No. 7,452,836 lies in the fact that crystals are grown throughout the whole volume without any spatial order leading to a homogenous material constitution. This is in contrast to a natural tooth and has important implications regarding both aesthetic and mechanical properties of the dental restoration, as shown in the following:
Natural teeth consist of a hard, inert and acellular enamel supported by the less mineralized, more resilient and vital hard tissue dentin. Because of its exceptionally high mineral content, enamel is a brittle tissue unable to withstand the forces of mastication without fracture unless it has the support of the more resilient dentin.
Enamel and dentin do not only differ in their mechanical and thermal properties, namely their compressive strength, elastic modulus and coefficient of thermal expansion, but also in their appearance. Whereas enamel is translucent and varies in colour from light yellow to grey-white, dentin is yellow. In a natural tooth, the thickness of enamel varies from a maximum of approximately 2.5 mm to a fraction thereof. This variation influences the tooth's appearance because the underlying dentin is seen through the thinner enamel region, whereas it gradually fades out towards thicker ones.
In summary, a natural tooth has thus an inhomogeneous structure different than in the glass-ceramic of U.S. Pat. No. 7,452,836 and can thus both with regard to aesthetic appearance and mechanical stability not be mimicked perfectly by the material disclosed in U.S. Pat. No. 7,452,836.
Aiming at a material mimicking the colour gradients in a natural tooth, WO 2010/010082 discloses a form-stabilized material comprising a first component and a second component, the second component having a different pigmentation than the first component and being disposed in the first component such that the boundary surface between the components represents a spatially curved surface. However, as a feldspathic ceramic, the material according to WO 2010/010082 does not exhibit suitable mechanical properties for indications such as multiple anterior or posterior unit-bridges.
Further, a method for manufacturing structured prostheses from a blank comprising at least one layer of high abrasive resistance, at least one layer of high flexural strength and at least one layer of lower hardness and strength is disclosed in U.S. Pat. No. 5,939,211. During the milling of the restoration, material removal is performed in such a manner that layers with high strength constitute a reinforcing structure.
In both the materials according to U.S. Pat. No. 5,939,211 and WO 2010/010082, the presence of physically distinct component layers and thus of an interface between different components has as a rule a negative impact on the overall stability of the dental restoration. Also, the methods according to these publications are relatively laborious.
Considering the drawbacks of the state of the art, it would be highly desirable to provide a process for the preparation of a prosthetic element comprising a glass-ceramic body, which allows precise adjusting of the properties to the actual need in a simple and effective manner.